1. Field of the Invention
The present invention relates to a diagnosing device for an evaporative emission control system, and particularly to a leak diagnosis of an evaporative emission control system containing a fuel tank.
2. Description of the Related Art
There is known an internal combustion engine that is equipped with an evaporative emission control system to prevent fuel evaporated in a fuel tank from being discharged to the atmosphere. In this system, evaporated fuel (evaporation gas) generated in the fuel tank is temporarily adsorbed to absorbent filled in a canister, and also the evaporated fuel thus adsorbed is discharged to an intake system of the internal combustion engine through a purge passage under a predetermined driving condition. However, when a part of this system is damaged for some reason, the evaporated fuel can be discharged into the atmosphere. In order to prevent such a situation, a leak diagnosis for judging the presence or absence of leakage of the evaporative emission control system is carried out (for example, see JP-A-2001-41116).
According to the leak diagnosis, a purge control valve (purge control solenoid valve) is fully closed, and then a canister closing valve (drain valve) is fully closed, whereby a hermetically sealed purge passage which is hermetically sealed under ambient pressure is formed and a pressure variation amount P1 caused by occurrence of fuel evaporated gas under positive pressure is measured. Thereafter, the purge control valve is opened to introduce intake pipe negative pressure into the fuel tank. When the inner pressure of the fuel tank reaches predetermined target inner pressure, the purge control valve is closed again, whereby the hermetically-sealed purge passage which is hermetically sealed under the predetermined negative pressure is formed, and a pressure variation amount P2 caused by occurrence of the evaporated fuel under the negative pressure is measured. The presence or absence of leak from the evaporative emission control system is judged on the basis of the relationship between the pressure variation amounts P1 and P2 thus measured.
However, according to the diagnosis method of hermetically sealing the evaporative emission control system under negative pressure on the basis of target inner pressure and judging the presence or absence of leak by using the pressure variation amount after the hermetical sealing under negative pressure, a negative pressure introducing time needed to set the inner pressure of the evaporative emission control system to the target inner pressure is longer as the generated amount of the evaporated fuel in the fuel tank is larger, so that the diagnosis time is lengthened.
Furthermore, in order to achieve a diagnosis result with high precision, it is needed to measure the pressure variation amount under a stable pressure state. However, the pressure state after the hermetical sealing under negative pressure is generally susceptible to introduction of negative pressure, and thus it trends to disrupt the stability as the negative pressure introducing time is longer, so that it is difficult to achieve a diagnosis result with high precision when a large amount of evaporated fuel occurs.